The invention concerns a method of paging in a cellular radio system including several cells, mobile stations and a database wherein location information of the mobile station is stored. In the method a first paging of the mobile station is performed in all cells of an area according to the stored location information.
In addition, the invention concerns an arrangement in a cellular radio system.
FIG. 1 in the appended drawing shows a block diagram of a simplified GSM mobile communications system. The Mobile Station MS is connected over the radio path with some Base Transceiver Station BTS, which is BTS1 in the example shown in FIG. 1. The Base Station System BSS comprises a Base Station Controller BSC and subordinated base transceiver stations BTS. Several Base Station Controllers BSC are usually subordinated to a Mobile Services Switching Centre MSC. The mobile services switching centre MSC is connected to other mobile services switching centres and through a Gateway Mobile Services Switching Centre GMSC the GSM network is connected to other networks, such as a public telephone network PSTN, another public land mobile network PLMN or an ISDN network. The operation of the whole system is monitored by an Operation and Maintenance Centre OMC. Subscriber information of the mobile station MS is stored permanently in the Home Location Register HLR and temporarily in that Visitor Location Register VLR in the area in which the mobile station MS is located at each time. The location information of the mobile station MS is stored in the visitor location register VLR with the accuracy of a Location Area LA.
The geographical area monitored by the visitor location register VLR is divided into one or more location areas LA, inside each of which the MS may move freely without notifying the VLR. One or more base transceiver stations BTS may operate within each location area. The base transceiver stations BTS send constant information about themselves and about their environment on their broadcast channel, such as a Base Station Identity Code BSIC, adjacent cell information and a Location Area Identifier LAI. Based on the last-mentioned, the MS which has locked into the base transceiver station BTS will know in which location area LA it is located at each time. If when changing base transceiver station BTS the mobile station notices that the location area identifier LAI of the base transceiver station has changed, the mobile station will send a location update request to the network. The location area of the mobile station MS will be updated in that visitor location register VLR, in the area of which the mobile station is located at each time. Information indicating in which VLR""s area the MS is located is relayed to the home location register HLR.
The mobile station""s location information stored in registers of the mobile communications system is used e.g. in routing of an incoming call. Based on location information of the mobile station MS available from the home location register HLR and from the visitor location register VLR, the incoming call is routed to that mobile services switching centre MSC in the area of which the mobile station MS of the called subscriber is located at each time according to the location information. The mobile services switching centre MSC asks its own visitor location register VLR for information on the called subscriber for setting up the call. The visitor location register VLR gives the required information on the mobile station MS including e.g. the mobile station""s MS location area LA stored in the latest location update, which is LA1 in the case shown in FIG. 1. Based on the location area information, the mobile services switching centre MSC will send a paging message provided with the mobile station""s MS identifier on the paging channels of all base transceiver stations of the location area in question, in the case shown in FIG. 1 on the paging channels of base transceiver stations BTS1, BTS2 and BTS3. Mobile stations MS located in the coverage area of base transceiver station BTS regularly receive the signal on Paging Channel PCH of the cell""s base transceiver station and they reply to the received paging on Random Access Channel RACH of that BTS, in the coverage area of which the mobile station MS is located when receiving, e.g. on the RACH channel of base transceiver station BTS1 in FIG. 1. On receiving the reply, the network reserves a traffic channel for the call and sets up a speech connection for mobile station MS.
When base transceiver stations BTS send a paging message, the mobile station MS may be momentarily in a shadow region of the network, e.g. in a tunnel, or mobile station MS does not receive the paging message correctly, e.g. due to interference or noise. The paging message will hereby not reach the called subscriber, even if the subscriber would otherwise be prepared to receive the call. Paging will also fail, if the mobile station MS is turned off unbeknownst to the mobile communications network or if the transmission power of mobile station MS is not sufficient for setting up a connection with the base transceiver station. If the mobile station does not reply to paging, e.g. within a certain period of time, it is generally known to perform re-paging of the mobile station MS over the entire location area LA through all base transceiver stations BTS in the location area. Typically, such re-paging is done no more than two or three times.
The publication xe2x80x9cAdaptive Paging Algorithms for Cellular Systemsxe2x80x9d, S. Madhavapeddy et al., Fifth WINLAB Workshop on Third Generation Wireless Information Networks, 1995, pages 347-361, presents the establishment and utilisation of a paging area based on statistics kept on the mobile station""s location. In the method according to the publication, paging of the mobile station is first done in the formed paging area, where the mobile station is expected to be located most probably. If the mobile station does not reply to the first paging, a new extended paging is performed, either once more in the same paging area or through all base transceiver stations in the location area. In another embodiment presented in the publication, paging of the mobile station is first done in the established paging area and next through all base transceiver stations of the system, if the mobile station does not reply to the first paging. If the mobile station does not reply even to the second paging, a third paging will be sent through those base transceiver stations which are not located within the established paging area.
It is a problem with known re-paging methods that due to the large paging area they load the network extensively, especially the limited resources of the radio path, and will cause plenty of signalling loads both on the radio path and between the radio network""s switching centre and base transceiver stations. Nor do known paging methods take into account that shadow areas of cells are of different size, whereby the need for re-paging will vary in cells. In addition, when the mobile station is turned off or if the transmission power of the mobile station is too low, re-paging will not be successful, whereby network resources have been spent on extended paging quite unnecessarily.
It is a purpose of this invention to implement optimum paging of a mobile station in a cellular radio system. In other words, the objective is paging of the mobile station with as few paging messages as possible, however, maintaining a good standard of service.
This new type of mobile station paging is achieved in the method according to the invention, which is characterised in that in the method the area according to the location information is divided according to the cell division into at least two sub-areas, each one of which is made up of at least one cell, limitation values for the number of pagings are defined on a sub-area basis for the cells and re-paging of the mobile station is performed so that the number of re-pagings to be performed in each area according to the location information is limited based on the said limitation values for the number of pagings.
The invention also concerns an arrangement in a cellular radio system comprising several cells, mobile stations and a database, wherein the mobile station""s location information is stored. According to the invention, the arrangement is characterised in that it comprises storing means for storing limitation values for the number of pagings in the cells and means for limiting re-pagings to be performed based on the said limitation values for the number of pagings.
The invention is based on the idea that re-paging of a mobile station is performed within an area which is smaller than the normal location area and wherein re-paging based on information collected from the network is most likely to be successful. Limitation values for the number of allowed pagings to be performed are defined separately for each cell or group of cells. Based on these limitation values, such cells are selected for each re-paging round wherein re-paging may still be performed. Re-paging is performed so many times in each cell as is indicated by the limitation value defined for the number of pagings for the cell, unless re-paging is found unnecessary e.g. if the mobile station has already replied to a sent paging message.
It is an advantage of such a paging method that it minimises network loading and, in particular, the use of radio resources in re-paging.
Another advantage of the paging method according to the invention is that re-paging may be targeted e.g. only at shadow areas of the network""s radio coverage or at poor signal areas of the network.
It is another advantage of the paging method according to the invention that in a radio network of a good quality, re-paging which is probably unnecessary is avoided, whereby information on failed paging can be forwarded more quickly, e.g. to subscriber A, or the call can be connected more quickly to the voice mail of the called subscriber.